Gas hydrates have been known for many years. These hydrates are inclusion compounds wherein various light hydrocarbon gases or other gases, such as natural gas, associated natural gas, methane, ethane, propane, butane, carbon dioxide, hydrogen sulfide, nitrogen, and combinations thereof, physically react with water at elevated pressures and low temperatures. The gas becomes included or entrapped within the extended solid water lattice network which includes hydrogen bonded water molecules. The hydrate structure is stable due to weak van der Waals' forces between the gas and water molecules and hydrogen bonding between water molecules within the lattice structure.
An exemplary, non-stoichiometric reaction equation for the formation of natural gas hydrates is as follows: EQU CH.sub.4 (g)+mH.sub.2 O(l).fwdarw.CH.sub.4 (H.sub.2 O)m(s).
In this equation, the value "m" typically is 4 to 6, and the heat of formation (.DELTA.H.sub.f) is -410 kJ/kg hydrate for methane hydrate, which is approximately 25% higher than the heat of fusion of water. The reverse reaction, exploited during regasification, is endothermic. Because gas hydrates are solids that form at a gas-water interface, the formation and regasification reactions are mass-transfer limited.
At least two different gas hydrate crystalline structures are known, each of which is a clathrate crystalline structure. A clathrate hydrate unit crystal of structure I includes two tetrakaidecahedron cavities and six dodecahedron cavities for every 46 water molecules. A clathrate hydrate unit crystal of structure II contains eight large hexakaidecahedron cavities and 16 dodecahedron cavities for every 136 water molecules. A relatively large volume of gas can be entrapped under pressure in these cavities. For example, it has been determined that natural gas hydrates can contain as much as 180 standard cubic feet of gas per cubic foot of the solid natural gas hydrates.
Early on, gas hydrates were considered an industrial nuisance. Petroleum and natural gas production facilities often are located in cold environments, where the product is located in deep underground or underwater wells. When tapping these wells, all of the necessary conditions and ingredients are present for producing gas hydrates--i.e., light hydrocarbon gases and water are present, the temperature is low, and the pressure is high. Therefore, gas hydrates often were produced spontaneously in the drilling and transmission pipes and equipment when an oil or natural gas well was tapped. Because gas hydrates are solid materials that do not readily flow in concentrated slurries or in solid form, when spontaneously produced in oil or natural gas production, they tend to clog the equipment, pipes, and channels in the production and transmission systems. These disadvantageous properties of gas hydrates spawned much research into methods for inhibiting hydrate formation and eliminating this nuisance. See, for example, D. Katz, et al., Handbook of Natural Gas, McGraw-Hill, New York (1959) pp. 189-221; and E. D. Sloan, Jr., Clathrate Hydrates of Natural Gases, Marcel Dekker, Inc. (1991). These documents are entirely incorporated herein by reference.
Because of the relatively high volume of gas that potentially can be stored in gas hydrates, however, eventually researchers began to look at this "nuisance" as a possible method for safely and cost effectively storing and/or transporting gases. See B. Miller, et al., Am. Gas. Assoc. Mon., Vol. 28, No. 2 (1946), pg. 63. This document is entirely incorporated herein by reference. Several researchers and patentees have described methods and systems for producing gas hydrates. See, for example, U.S. Pat. No. 3,514,274 to Cahn, et al., which document is entirely incorporated herein by reference.
Gudmundsson describes various systems for producing gas hydrates. See, for example, U.S. Pat. No. 5,536,893; WO Patent Publication No. 93/01153; "Transport of Natural Gas as Frozen Hydrate," ISOPE Conference Proceedings, V1, The Hague, Netherlands, June 1995; and "Storing Natural Gas as Frozen Hydrate," SPE Production & Facilities, February 1994. These documents each are entirely incorporated herein by reference. In these documents, Gudmundsson discloses storing gas hydrates under "metastable" conditions, i.e., conditions under which one would normally expect the hydrates to be unstable and decompose. For example, in U.S. Pat. No. 5,536.893. Gudmundsson describes agglomerating gas hydrates into solid blocks suitable for long term storage at atmospheric pressure and at a temperature below 0 to -15.degree. C. One would expect the hydrates to be unstable or decompose under these metastable conditions because these temperature and pressure conditions are not suitable for gas hydrate formation. Under relatively mild metastable conditions (e.g., 5 to 20.degree. F. and ambient pressure), however, gas hydrates dissociate sufficiently slowly to remain intact for periods of time suitable to ocean transport or large-scale storage (e.g., for 10 days or more). This metastability phenomenon is attributed, at least in part, to spontaneous regasification of the outer surface of a macroscopic hydrate sample. Because the hydrate regasification process is endothermic, once the outer surface of the hydrate sample dissociates, auto-refrigeration freezes the dissociated water to create an ice shell that significantly insulates the bulk hydrates and attenuates the mass transfer rate of gas from within the interior of the sample.
Additionally, gas hydrates are effective insulators (thermal conductivity "k" of about 0.5 W/m K for hydrates, as compared to a thermal conductivity "k" of 2 for ice). This insulative property protects the interior gas hydrates in a bulk sample from heating and helps keep bulk gas hydrates from dissociating too rapidly. Thus, the metastability and insulative properties of gas hydrates allow them to remain stable under relatively mild conditions after they are initially produced.
Traditionally, hydrate-forming gases, such as natural gas, associated natural gas, methane, ethane, propane, butane, carbon dioxide, nitrogen, and hydrogen sulfide, have been stored under high pressures. Liquefied-natural gas ("LNG") and liquefied propane gas ("LPG") are examples of this type of storage system. Because of the need for high pressure cylinders, storage of gases under high pressures and liquefied conditions presents significant safety issues and is very expensive.